Nucleic acids are used in various forms in various fields. For example, in a field of recombinant nucleic acid technology, it is required that nucleic acids are used in the form of probes, genomic nucleic acids and plasmid nucleic acids.
Nucleic acids are also used in various methods in a diagnostic field. For example, nucleic acid probes are routinely used for the detection and diagnosis of human pathogens. Similarly, nucleic acids are used for detecting genetic disturbances. In addition, nucleic acids are also used for detecting food-contaminating substances. Further, nucleic acids are routinely used for the localization, identification and isolation of nucleic acids of interest for various reasons such as mapping, cloning and recombination expression.
In many cases, nucleic acids can only be obtained in a small quantity, and isolation and separation operations thereof are complicated and require time. These often time-consuming, complicated operations tend to lead to loss of nucleic acids. In the purification of nucleic acids in samples obtained from blood sera, urine and cultures of bacteria, the risk of resulting in contamination and a false positive will be added.
One of the widely known purification methods is a method for purifying nucleic acids by adsorbing them on the surface of silicon dioxide, silica polymers, magnesium silicate or the like, followed by operations such as washing, desorption and the like (for example, JP Patent Publication (Kokoku) No. 7-51065 B (1995)). This method is excellent in separation performance. However, industrial mass production of the adsorbents with the same performance is difficult, and also there are problems that these adsorbents are inconvenient in handling and difficult to be processed into various shapes; the method using centrifugal separation is difficult to be automated; and the like.
Moreover, a well known method for purifying nucleic acid includes a method in which nucleic acids are adsorbed on or desorbed from the solid phase by pressurizing the inside of a syringe by a piston or a pump. According to conventional methods, the inside of a syringe is pressurized and the liquid in the syringe is discharged to the outside of the syringe after a lapse of a certain time, and then next operation has been performed. However, there is a problem that the time for extruding all of the liquid in the syringe to the outside of the syringe varies depending on the properties of the liquid (particularly, viscosity of the liquid or the like). Namely, when a liquid having high viscosity is used, it takes relatively long time to extrude total amount of the liquid in the syringe. On the other hand, for a liquid having low viscosity, total amount of the liquid in the syringe can be extruded in a shorter time. Therefore, there has been a problem that when nucleic acids are simultaneously purified from many types of specimens using an automated apparatus, the operation for ensuring the discharge of the total amount of the specimen in the syringe for all kinds of liquid takes a long time, because it is necessary to set enough time to allow the total amount to be extruded even for the specimen having the highest viscosity. Further, even if enough time is set, there has been no means to prove the fact that the total amount of the specimen has been discharged without fail. Furthermore, for the automation of the apparatus, it has been necessary to monitor the fact that the total amount of the specimen in the syringe has been discharged.
Recently, there has been made an attempt that nucleic acids are separated and purified by performing the steps of (1) pressurizing a nucleic acid-containing sample solution in a container and passing the above described sample solution through a solid phase located in the container to adsorb the nucleic acid on the solid phase, (2) adding a washing solution into the container and pressuring the above described washing solution to pass through the solid phase to wash the solid phase, and (3) adding a liquid for desorbing the nucleic acid from the solid phase into the container and pressurizing the above described liquid to pass through the solid phase to recover the nucleic acid into the above described liquid. The solid phase for use in the method may include, for example, a glass filter, an organic polymer having a hydroxyl group on the surface thereof or the like. In the above steps (1) to (3), pressurizing allows the liquid in the container to be passed through the solid phase to be discharged to the outside of the container, wherein the pressure inside the container has been ever increasing by the pressurizing. However, too high pressurizing causes too high rate for extruding the liquid in the container. This leads to a problem that the liquid forms droplets in the container and the liquid in the form of droplets is left in the container.